In specific installations, such as a heating installation for example, it is necessary that two of the elements that come to form part of the operation must be associated, as far as hookup and start are concerned, in such a way that one of them should always start operating after the other, and the latter should not disconnect after the second element.
In this sense, the electric resistor and the ventilator of a heating installation can be cited as an example. The resistor should not be connected without the corresponding ventilator being activated. In other words, the connecting of the electric resistor when the ventilator is not operating should be prevented, while when the ventilator is operating activation of the electric resistor is optional and, in the connecting position of the electric resistor, the ventilator cannot be disconnected.
In such a case it is convenient that the switch of the electric resistor is subjected to the active position of the ventilator switch.